Universe 8e Lecture Chapter

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Roger A. Freedman William J. Kaufmann III
Universe Eighth Edition
CHAPTER 10 Our Barren Moon
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By reading this chapter, you will learn

• 10-1 The nature of the Moons surface
• 10-2 The story of human exploration of the Moon
• 10-3 How we have learned about the Moons
  interior
• 10-4 How Moon rocks differ from rocks on Earth
• 10-5 Why scientists think the Moon formed as the
  result of a violent collision between worlds

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Comparing the Earth and Moon (a) The Moon is
a bit more than one-quarter the diameter of the
Earth. The average Earth-Moon distance is about
30 times the Earths diameter.
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Comparing the Earth and Moon The Earth has
blue oceans, an atmosphere streaked with white
clouds, and continents continually being
reshaped by plate tectonics. By contrast, the
Moon has no oceans, no atmosphere, and no
evidence of plate tectonics.
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Motion of the Earth-Moon System The Earth and
Moon both orbit around their center of mass,
which in turn follows an elliptical orbit around
the Sun.
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The Moon as Seen from Earth The key features of
the lunar surface can be seen with binoculars, a
small telescope, or even the naked eye
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The Crater Clavius This photograph from the
5-meter (200-in.) Palomar telescope shows one of
the largest craters on the Moon. Clavius has
a diameter of 232 km (144 mi) and a depth of 4.9
km (16,000 ft), measured from the craters floor
to the top of the surrounding rim
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Details of Mare Tranquillitatis This color
photograph from lunar orbit reveals numerous
tiny craters in the surface of a typical mare.
The curving river, or rille, was carved out by
flowing lava that later solidified similar
features are found in volcanic areas on Earth.
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The formation of craters on the Moon
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Apollo 12 landed on Moon
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Iron on the Moon Images at different
wavelengths from the Clementine spacecraft were
used to make this map of the concentration of
iron on the lunar surface. The areas of highest
concentration (red) coincide closely with the
maria on the near side (compare Figure 10-7),
confirming that the maria formed from iron-rich
lavas. The lowest iron concentration (blue) is
found in the lunar highlands. (No data were
collected in the gray areas.) The large green
region of intermediate iron concentration on the
far side is the South PoleAitken Basin
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The Internal Structure of the Moon Like the
Earth, the Moon has a crust, a mantle, and a
core. The lunar crust has an average thickness of
about 60 km on the Earth-facing side but about
100 km on the far side. The crust and solid upper
mantle form a lithosphere about 800 km thick. The
plastic (nonrigid) asthenosphere extends all the
way to the base of the mantle. The iron-rich
core is roughly 700 km in diameter.
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The Regolith Billions of years of bombardment
by space debris have pulverized the uppermost
layer of the Moons surface into powdered rock.
This layer, called the regolith, is utterly
bone-dry. It nonetheless sticks together like wet
sand, as shown by the sharp outline of an
astronauts bootprint.
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The gravitational force between any two objects
decreases with increasing distance between the
objects. This principle leads to a simple
formula for estimating the tidal force that the
Earth exerts on parts of the Moon. Consider two
small objects, each of mass m, on opposite sides
of the Moon. Because the two objects are at
different distances from the Earths center, the
Earth exerts different forces Fnear and Ffar on
them (see the accompanying illustration). The
consequence of this difference is that the two
objects tend to pull away from each other and
away from the center of the Moon (see Section
4-8, especially Figure 4-23). The tidal force on
these two objects, which is the force that tends
to pull them apart, is the difference between the
forces on the individual Objects.
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specimen of mare basalt was brought back by the
crew of Apollo 15.
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Anorthosite The light-colored lunar terrae
(highlands) are composed of this ancient type of
rock, which is thought to be the material of
the original lunar crust. Lunar anorthosites vary
in color from dark gray to white this sample
from the Apollo 16 mission is a medium gray.
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Anorthosite The light-colored lunar terrae
(highlands) are composed of this ancient type of
rock, which is thought to be the material of
the original lunar crust. Lunar anorthosites vary
in color from dark gray to white this sample
from the Apollo 16 mission is a medium gray.
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The Rate of Crater Formation on the Moon This
graph shows the rate at which impact craters
formed over the Moons history.
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The Moons Tidal Recession The Earths rapid
rotation drags the tidal bulge of the oceans
about 10 ahead of a direct alignment with the
Moon. The bulge on the side nearest the Moon
exerts more gravitational force than the other,
more distant bulge. The net effect is a small
forward force on the Moon that makes it spiral
slowly away from the Earth. Tidal interactions
between the Earth and Moon are slowing the
Earths rotation and pushing the Moon away from
the Earth.
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The collisional ejection theory of the Moons
origin holds that the proto-Earth was struck by a
Mars-sized protoplanet and that debris from this
collision coalesced to form the Moon. This theory
successfully explains most properties of the Moon.
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Key Ideas

• Appearance of the Moon The Earth-facing side of
  the Moon displays light-colored, heavily cratered
  highlands and dark-colored, smooth-surfaced
  maria. The Moons far side has almost no maria.
• Virtually all lunar craters were caused by space
  debris striking the surface. There is no evidence
  of plate tectonic activity on the Moon.

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Key Ideas

• Internal Structure of the Moon Much of our
  knowledge about the Moon has come from human
  exploration in the 1960s and early 1970s and from
  more recent observations by unmanned spacecraft.
• Analysis of seismic waves and other data
  indicates that the Moon has a crust thicker than
  that of the Earth (and thickest on the far side
  of the Moon), a mantle with a thickness equal to
  about 80 of the Moons radius, and a small iron
  core.

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Key Ideas

• The Moons lithosphere is far thicker than that
  of the Earth.
• The lunar asthenosphere probably extends from the
  base of the lithosphere to the core.
• The Moon has no global magnetic field today,
  although it had a weak magnetic field billions of
  years ago.

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Key Ideas

• Geologic History of the Moon The anorthositic
  crust exposed in the highlands was formed between
  4.3 and 4.0 billion years ago.
• An era of heavy bombardment formed the maria
  basins between 4.0 and 3.8 billion years ago, and
  the mare basalts solidified between 3.8 and 3.1
  billion years ago.
• The Moons surface has undergone very little
  change over the past 3 billion years.

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Key Ideas

• Meteoroid impacts have been the only significant
  weathering agent on the Moon.
• The Moons regolith, or surface layer of powdered
  and fractured rock, was formed by meteoritic
  action.
• All of the lunar rock samples are igneous rocks
  formed largely of minerals found in terrestrial
  rocks.
• The lunar rocks contain no water and also differ
  from terrestrial rocks in being relatively
  enriched in the refractory elements and depleted
  in the volatile elements.

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Key Ideas

• Origin of the Moon The collisional ejection
  theory of the Moons origin holds that the
  proto-Earth was struck by a Mars-sized
  protoplanet and that debris from this collision
  coalesced to form the Moon. This theory
  successfully explains most properties of the
  Moon.
• The Moon was molten in its early stages, and the
  anorthositic crust solidified from low-density
  magma that floated to the lunar surface. The mare
  basins were created later by the impact of
  planetesimals and filled with lava from the lunar
  interior.
• Tidal interactions between the Earth and Moon are
  slowing the Earths rotation and pushing the Moon
  away from the Earth.